Decode DGPS Transmissions With Your iPhone and iPad

This app can decode DGPS (Differential Global Positioning System) beacon transmissions. DGPS stations transmit the difference between positions indicated by GPS satellite systems and the known fixed position of the station. This allows higher accuracy. DGPS transmissions are 100 or 200 baud and are transmitted on frequencies from 285 kHz to 325 kHz. Hundreds of these stations are operated by the Coast Guard and other agencies, and they can be interesting DX targets.

The app uses the microphone to listen to audio from the radio, so be sure you gave it permission when you ran the app the first time. If you didn't, you can go to the Privacy Settings for your device, tap Microphone on the right side, then make sure access us turned on for this app. Likewise, you want to give it permission to use Location Services, so it can set your location, for use when computing the distance to received beacons.

To use this app, you need a radio that can tune the correct frequency range, either in SSB or CW mode. Most “shortwave radios” can do this. Ideally you use a cable to feed the audio from your radio into your device, but you can also place the radio speaker near your device's microphone, or even better yet, plug earphones into the radio, and place them next to the device's microphone. You may need to experiment to get the best quality audio pickup, without any outside noise or distortion.

To decode the transmission, tune your radio to a DGPS frequency. You can either tune directly to the frequency in CW mode, in which case you set the center frequency in this app to frequency of the tone produced by your radio in CW mode, usually close to 800 Hz, or use USB mode, tune 1 kHz low, and set the center frequency in this app to 1000 Hz. The center frequency is set by tapping Settings, and changing it there. There is a display of the audio spectrum, to help you with tuning, if necessary.

While there, also be sure to set your location so that the correct distance and bearing is calculated. You can directly enter your longitude and latitude (negative for west and south) or tap Set Location and it will be set based on where your device thinks you are located.

Select the baud rate, either 100 or 200 baud, tapping the button in the upper left part of your device's screen. Eventually, if you have tuned into a DGPS transmission that is strong enough, you will start seeing decode messages printed.

First the current UTC date and time are printed in brackets.
Then the station ID, 806 in this case, as well as the two reference IDs, 12 and 13 in this case.
That is followed by the assigned frequency, 289.0 kHz for this station. You can compare this value to what your radio is tuned to, they should match. If they don't, then ignore this message, as it was incorrectly received. With weak signals, it is common to receive incorrect messages, because static or other interference has corrupted some of the bits. Just ignore them.
The location of the station is then displayed, the city, state, and country, followed by the station location.
Then the distance and bearing to the station is displayed, providing you have correctly set your location in the app.

Settings will also let you change the size of the displayed text. You can also clear the received text here, or clear it by shaking your device.

Recording mode can be turned on in Settings, any text that is displayed will be written to a text file that you can later view, or transfer to your computer using File Sharing in iTunes. You can select which recording file to append additional text to, or create a new file.